I understand that the Pontiac Vibe and the Toyota Matrix are similar cars, but why do some prefer the Pontiac Vibe, since it's an American car, and less reliable?

Some just don't CARE about anything but what a car LOOKS like when they buy. These people usually change cars every year or two, and don't concern themselves with petty things like interest rates, reliability records, price etc.

NO i vote for the Toyota Matrix i used to have one a red one

They are the same cars
Same engine, transmission, chassis, suspension. Same reliability abd gas mileage and HP.
Pontiac makes the Toyota Matrix for Toyota or vice versa.

The Pontiac Vibe looks better. Sportier looks.